To provide a new pump technique for thickening muddy water continuously.

A plunger pump is constituted by housing a weir ring 2d-fit piston 2 in a water-permeable cylinder 3 consisting of a water-permeable material. Muddy water Mw is sucked in a cylinder head chamber Ch by elevating the piston 2. When the sucked muddy water Mw is pressurized by lowering the piston 2, clear water being a component of the water Mw moves to a clear water chamber 5. The filtered mud particle is scraped by the ring 2d of the piston 2 and diluted with a large quantity of the water Mw. The concentration of the mud particle-dissolved water Mw is increased. In other words, the water Mw is thickened.